A current relay is a device that is often used to signal an overcurrent in a specific controlled circuit. It is also used to disconnect an electrical circuit in the event of short circuits or overloads. The minimum current relay is used much less frequently. Such devices are designed to break the electric circuit when a certain minimum current value is reached.
There are many different types of electrical devices such as current relays. They differ in their design and principles of action. If we talk about the so-called classic device, then it is a movable anchor on the springs, which controls the contacts, and a coil with a core (usually iron). When current flows through the coil, a magnetic field of a certain magnitude is created. The core of the coil is magnetized by the action of this magnetic field and begins to attract the armature. Thus, contacts will be triggered.
The coil of such a device contains a few turns, however, the wire has a large diameter (unlike, for example, the same voltage relay). The diameter of the wire directly depends on the current, more precisely, on the value of the rated current. Due to this, a certain voltage drop is achieved. This is very important, because the coil is connected in series in a controlled circuit.
Some DC relays have an adjustable trip current. Most often this is achieved by changing the tension of the spring of the armature. The AC current relay , which is used to control high currents, can be activated via transformers.
The most important characteristic of such a protective device is the response time. Devices of this type, which can be used to protect against short circuits, have a response time of not more than several tens of milliseconds.
A solid state DC relay delays when the circuit is disconnected. This eliminates the possibility of false alarms in the event of a short-term increase in current. Such a device usually has a response time adjustment.
One of the most common types of protective devices is a
thermal current
relay . It is a
bimetallic plate equipped with a heating element made of a material with a high resistivity value (for example, nichrome). The plate consists of materials with different coefficients of thermal expansion, which bends when heated and acts on the relay mechanism. The response time of such a device depends on the current - the larger it is, the faster the plate warms up, and the shorter the response time.
An electronic current relay is used for virtually instantaneous tripping during overload. The electronic relay circuit can process the signals in accordance with predefined parameters and characteristics. You can set the permissible maximum current and delay time during shutdown. Such current relays can be both variable and constant. These devices are often integrated into many devices.